The role of the gonadal steroids on the morphogenetic aspects of sexual differentiation of the neonatal gonadotrophic axis will be further defined in long-term, organotypic cultures of the rodent hypothalamus plus or minus pituitary. The cytological parameters of differentiation and development will be studied in living and stained preparations at the light and scanning electron microscopic levels and correlated with biochemical studies of the steroid effects. The following areas will be investigated: 1. Further characterization of the observed responses to steroid to determine the roles of estrogen and androgen during the "critical period" and with maturation; 2. Identification of the steroid-sensitive cells in order to study their specific ontogeny and their relationship to the LH-RH and dopamine neurons of the hypothalamus; 3. Metabolic aspects of the steroids and their receptors. Information will also be obtained by radioimmunoassay, fluorescence and immunohistochemistry and by 3H-steroid autoradiography. This unique model can provide new information, heretofore unobtainable by other methods, on the developmental aspects of hypothalamic regulation of gonadotrophin secretion and the way by which the sex hormones modify these regulatory processes. These aspects are critical to an understanding of the neural substrate of sexual differentiation of the brain and of the neural control of the reproductive processes proper.